ABHANDLUNGEN DER GEOLOGISCHEN BUNDESANSTALT Abh

Home , Greywacke zone, Moldanubian Zone, Saxothuringian Zone

©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at

ABHANDLUNGEN DER GEOLOGISCHEN BUNDESANSTALT Abh. Geol. B.-A. ISSN 0016–7800 ISBN 3-85316-02-6 Band 54 S. 7–13 Wien, Dezember 1999

North Gondwana: Mid-Paleozoic Terranes, Stratigraphy and Biota Editors: R. Feist, J.A. Talent & A. Daurer

Tectonic and Plate Tectonic Units at the North Gondwana Margin: Evidence from the Central European Variscides

WOLFGANG FRANKE*) 2 Text-Figures

Variscan Belt Central Europe Terranes

Contents

Zusammenfassung ...... 7 Abstract ...... 7 Text ...... 8 References ...... 12

Tektonische und plattentektonische Einheiten am Nordrand von Gondwana: Hinweise aus mitteleuropäischen Varisziden Zusammenfassung Für die Zeitscheibe des Mitteldevons lassen sich im mitteleuropäischen Variszikum die folgenden plattentektonischen Einheiten unter- scheiden: ● Old Red-Kontinent, bestehend aus Laurentia + Baltica + Avalonia. Am Südrand von Avalonia waren akkretioniert: • ein silurischer Inselbogen (das Resultat der Schließung des Rheischen Ozeans), und • „gestrandete“ armorikanische Fragmente ● Schmaler Rhenohercynischer Ozean ● Saxothuringisches Terrane (oder Terranes), bestehend aus der späteren Mitteldeutschen Kristallinschwelle und dem saxothuringischen Becken ● Schmaler Saxothuringischer Ozean ● Bohemia (oder Perunica) ● Schmaler Moldanubischer Ozean ● Gondwana (oder aber ein weiteres Fragment des Gondwana-Nordrandes).

Abstract In Middle Devonian time, the following plate-tectonic units can be recognized in the Variscides of Central Europe: ● Old Red Continent, consisting of Laurentia + Baltica + Avalonia, with •a Silurian arc (resulting from the closure of the Rheic ocean) and • stranded Armorican fragments accreted to its southern margin. ● Rhenohercynian narrow ocean ● Saxothuringian terrane or terrane(s) (basement of the Mid-German Crystalline Rise and basement of the Saxothuringian basin) ● Saxothuringian narrow ocean ● Bohemian terrane (or Perunica) ● Moldanubian narrow ocean ● Gondwana mainland (or another Gondwana-related terrane).

*) Authors address: WOLFGANG FRANKE: Institut für Geowissenschaften, Justus-Liebig-Universität, Senckenbergstraße 3, D-35394 Giessen. e-mail: [email protected].

7 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at

Identification of plate tectonic units (continents, mi- sif, and its equivalents in the Harz Mts. along-strike to the croplates, “terranes“) and reconstruction of their migra- NE. These rocks represent a narrow Rhenohercynian tion and assembly is a complex task. It requires all kinds ocean (i.e., the drift stage of Devonian extension). Equi- of geological evidence, combined with palaeomagnetic, valents of the Giessen nappe and its basal Armorican biogeographic and palaeoclimate data. These latter slices are exposed in the Lizard and Carrick nappes of methods have large error bars, and mostly fail to reveal south Cornwall. minor features. Hence, any attempt to summarize the All these Rhenohercynian lithologies permit to deduce state of the art has to start from geological field evidence. the following plate tectonic development: subsequent to Therefore, the following abstract is “extended“, and has the formation of the Old Red Assembly (Avalonia + Baltica to be based upon the recognition of tectonostratigraphic + Laurentia), convergence between the Old Red and Ar- units in the Central European segment of the Variscan Belt morica produced an island arc, which was accreted to the (as they are seen today). These units are then translated, southern margin of the Old Red Continent. Continued with the aid of complementary data, into plate tectonic convergence added Armorican fragments. All these units entities. It is vital to distinguish between these two cate- were overstepped by Devonian shelf sediments. Further gories, which are often mixed up with each other. Major south, a narrow oceanic domain was developed. South- and minor tectonostratigraphic units (marked as bold ward subduction of the Rhenohercynian ocean com- and underlined respectively) are briefly described in order menced in the Frasnian, and was followed by underplat- from N to S; palaeogeographic interpretations are ing of pre-Devonian basement from the Late Tournaisian shown in bold italics. The locations of the tectonostrati- onwards. Parts of the oceanic domain were transported graphic units are shown in Text-Fig. 1; the plate kinematic as thrust sheets on to the northwestern foreland, carrying evolution is summarized in a cartoon (Text-Fig. 2). with them Devonian and pre-Devonian rocks of the distal The Rhenohercynian belt represents a passive plate passive margin (FRANKE & ONCKEN, 1995). margin, which originated from crustal extension in Early The Mid-German Crystalline High (MGCR) represents Devonian through to Early Carboniferous time. the source area for the synorogenic clastic sediments The basement of the Devonian sequences is compo- (active margin) deposited on the Rhenohercynian fore- site: Most of the extended continental crust belongs to land in Frasnian through to Late Carboniferous time. Pre- the Avalonian microplate, which forms the southern- sent day exposures in the Odenwald, Spessart and most element of an assembly known as the “Old Red Thuringian Forest are mainly composed of arc-related Continent”: the Old Red composite plate resulted from plutonic rocks of Late Devonian to Early Carboniferous the amalgamation of Avalonia with Baltica (in Silurian age (363–335 Ma). Magmatic arc rocks of Silurian age are time), and Avalonia/Baltica with Laurentia (accomplished exposed in the basement of the Permian Saar/Nahe basin in the Silurian/Early Devonian). This Avalonian basement W of the River Rhine, in the Spessart and in the Thuringian in the Rhenohercynian belt was formed or reactivated by a Forest. These are equivalents of the Silurian arc exposed Cadomian event between ca 600 and 540 Ma. Rocks of in the Phyllite Zone at the southern margin of the RH, but this age are exposed in the Ecker gneiss (a roof pendant in were tectonically underplated under the MGCR and later the late Carboniferous Brocken granite of the Harz Mts.) exhumed in the core of an antiform within the upper plate and in the Wartenstein gneiss at the S margin of the Rhe- (ONCKEN, 1997). For most of the metamorphic rocks con- nohercynian west of the River Rhine. There is no indica- tained in the MGCR, protolith ages and plate tectonic affi- tion of the latest Ordovician Saharan glaciation, and Or- liations are unknown, and relationships with areas further dovician volcanics in the British part of Avalonia (Wales) S are therefore uncertain. show intermediate palaeo-latitudes. The Saxo-Thuringian belt is a Cambro-Ordovician A more southerly part of the pre-Devonian basement is rift basin developed on Cadomian crust. Late Proterozoic exposed in the Saar/Nahe region west of the Rhine and in sediments with Cadomian deformation and low-grade the Taunus Mts. at the southern margin of the RH east of metamorphism are exposed in the core of the Schwarz- the Rhine (Phyllite Zone). These areas contain Silurian burg anticline and occur as pebbles within locally derived magmatic rocks (now in greenschist facies) whose geo- Frasnian conglomerates. Late Cadomian (550–540 Ma) chemical fingerprints indicate an active margin (arc) granitoids are documented in the Erzgebirge antiform and setting. This magmatic belt is taken to represent closure in the Lausitz (Lusatia, E of the Elbe Fault Zone). A second of the Rheic ocean between Avalonia and Armorica. pulse of rifting in the Frasnian is documented by bimodal Rocks of the same age occur as pebbles in the Carbonif- intra-plate volcanic rocks, and clastic sediments derived erous flysch of south Cornwall (DÖRR et al., 1999). from local halfgraben shoulders. The southernmost (i.e., the most distal) part of the Rhe- The basin was overthrust from the SE, by the active, nohercynian passive margin is locally preserved in tec- northwestern margin of the Tepla-Barrandian terrane (see tonic slices at the base of thrust sheets derived from the below). This is documented by tectonic klippen (Münch- south. These are Early Devonian debris flow deposits and berg, Wildenfels, Frankenberg) which contain (from top to turbidites with clasts of Ordovician quartzites and Silurian bottom) metamorphosed Tepla-Barrandian rocks, eclo- to Early Devonian limestones, all of which have faunas of gites derived from ca 500 Ma MOR-type mafic rocks, and “Bohemian” (i.e., Armorican) affinities. Ordovician accreted slope sediments of the Saxo-Thuringian margin. quartzites (?Armorica) are also present at the southern The par-autochthon is characterized by shelf and he- margin of the Rhenohercynian belt in the Harz Mts. mipelagic sediments. The continental crust of the par-au- The thrust sheets carrying these Armorican fragments tochthon represents a Saxo-Thuringian terrane. The are mainly composed of Early Devonian MOR-type me- continental source, which should be expected to the NW tabasalts, overlain by condensed deep-water pelites and of the shelf, has not been preserved: paradoxically, the synorogenic greywackes of Frasnian through to Early shelf is bounded to the NW by a belt with early Palaeozoic Carboniferous age. Assemblages of this kind are known bimodal magmatic rocks (KEMNITZ et al., in press; Vesser from the Giessen nappe in the E part of the Rhenish Mas- unit), which represent another crustal-scale neck (the

8 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at same problem is apparent from the Ordovician facies dis- The NE margin of the Bohemian Massif is segmented by tribution in the Central Armorican zone and in the Central NW-trending, dextral strike-slip faults, of which the Elbe Iberian zone (ROBARDET & GUTIERREZ-MARCO, 1990). and the Intrasudetic fault zones are the most important. Eclogite facies metamorphism in the Saxothuringian These latter fault zones contain, between them, an ex- nappes predates 380 Ma Emplacement and refolding of tremely complex association of rocks summarized as the the nappes occurred in latest Viséan time (330–325 Ma). W-Sudetes. Into this “megabreccia”, OLIVER et al. (1993) The approach of the tectonic front is heralded by Frasnian have read an amalgamation of individual terranes, which through to Late Viséan synorogenic clastic sediments. are supposed to include the main (“Caledonian“) suture Their composition contains a complete record of uplift between Baltica and Gondwana. However, FRANKE et al. and exhumation of the southeasterly sources (SCHÄFER et (1993) and ZELAZNIEWICZ & FRANKE (1994) have identified, al., 1997). within the Sudetes, NE-ward continuations of tectonic The Saxonian granulites (type locality of the metamor- units known from the more westerly part of the belt, dis- phic facies!) are exposed in a metamorphic core complex placed by the major dextral faults cutting across the main within the Saxothuringian par-autochthon. Similar HP and structural trend. This latter view is backed up by proven- HP/HT rocks occur in the Erzgebirge to the SE. These ance studies of Cambrian clastic sediments (BELKA et al., rocks are enigmatic, since isotopic studies suggest high 1997): K-Ar ages of detrital micas reveal that the bound- grade metamorphism in Late Devonian/Early Carbonifer- ary between Gondwana-related terranes and Baltica lies ous time, i.e., during the time of deposition of the overly- not only to the NE of the Sudetes, but even NE of the Holy ing low-grade sequences. If the isotopic data are correct, Cross Mts. the high grade rocks must have been emplaced under the The SE margin of the Bohemian Massif is complicated floor of a subsiding marine basin (Dekorp and Orogenic by the NE-trending “Moldanubian thrust”, a first-order Processes Research Groups, in press). zone of dextral transpression which cuts off (i.e., post- The eclogites of the Münchberg nappe are derived from dates) the Elbe and Intrasudetic fault zones. The a separate tectonic unit at the NW margin of the Tepla- “E-Sudetes” SE of the fault zone contain Cadomian Barrandian (see below), the Marianske Lazne meta- basement, overlain by Devonian platform and rift, and ophiolite. These metabasic units, both formed around Carboniferous foreland basin sequences. These rocks re- 500 Ma, represent thinned continental crust or a narrow present an eastward continuation of the Rhenohercynian Saxothuringian ocean separating the Saxothuringian belt (i.e., Avalonia), which has been bent into an orocline from the terranes adjacent to the south. encircling the Bohemian massif in the E. Clockwise rota- The main part of the Tepla-Barrandian unit forms the tion of the E-Sudetes through ca 90° has produced a core of the Mid-European Variscides, and the zone of “Bohemian arc” proven by palaeomagnetic studies (TAIT structural parting between the N and S flanks of the belt. It et al., 1996). is floored by Late Proterozoic sedimentary and volcanic Taken altogether, the tectonic units of the Central Euro- rocks with Cadomian deformation and low-grade meta- pean segment of the Variscides represent remnants of Av- morphism dated at ca 540 Ma. This pre-Variscan base- alonia and the Armorican terrane family. The boundary ment is intruded by bimodal intraplate magmatic rocks against Gondwana mainland is uncertain for several rea- dated at 520–480 Ma (DÖRR et al., 1998; ZULAUF et al., sons: 1997), and unconformably overlain by Cambrian through – In the Moldanubian zone, high grade metamorphism to Givetian sediments with a weak Variscan overprint. This Palaeozoic cover sequence records moderate Cam- has obliterated any palaeogeographic clues; bro-Ordovician extension and a Devonian epicratonic – The “Bohemian arc” and the “Moldanubian thrust” have stage. The age of the extensional phase matches that of displaced rocks from the N flank of the Variscan orogen the Saxo-Thuringian belt. However, palaeomagnetic towards its SE margin. Therefore, we do not know, studies have revealed that the Tepla-Barrandian has been which terranes were originally adjacent to the units now rotated counter-clockwise through more than 140° after contained within the Moldanubian region. the Silurian and before the Middle Devonian, whereas the – All areas SE of the Bohemian massif have been strongly Saxo-Thuringian terrane has maintained its orientation reworked and displaced during the Alpine cycle. A re- (TAIT et al., 1997). Hence, the Tepla-Barrandian must re- cent summary of the Pre-Alpine basement of the Alps is present a separate microplate (Bohemian terrane or available in VON R AUMER et al. (eds., 1993). According to Perunica) within an Armorican terrane family (archi- SCHÄTZ et al. (1996), the Variscan basement of the pelago). Northern Greywacke Zone (traditionally regarded as The Moldanubian (sensu stricto) to the W and S of the part of the Adriatic plate), in Silurian time, was situated Tepla-Barrandian is entirely made up of high grade meta- at 50°S, as compared with 21°S and 28°S obtained for morphic rocks, whose protolith ages and palaeogeo- the Saxothuringian and Tepla-Barrandian, respective- graphic affinities are largely enigmatic. The Gföhl unit, ly. By the Devonian, this area had attained 30°S (SCHÄTZ composed of HP and HP/HT rocks, has been thrust to- et al., 1997), which is not significantly different from the wards the E/SE over the LP/HT rocks of the Drosendorf palaeolatitude of the Saxothuringian and Rhenoher- unit. The high grade nappes are usually taken to reflect cynian. If the South Alpine basement represents subduction and obduction of rifted or even oceanic crust, Gondwana, a major oceanic separation between Gond- representing a Moldanubian basin or narrow ocean. wana and the more northerly terranes either has never This is tentatively correlated with the Massif Central existed, or else must pre-date the Devonian. Alternat- ocean identified in the French segment of the orogen. ively, the South Alpine basement might represent a Hence, the continental crust of the Drosendorf unit might separate peri-Gondwanian fragment. represent Gondwana mainland. Closure of the Moldanu- For future studies, palaeogeographic units on the N bian-Massif Central ocean by northward subduction flank of the Variscan belt should be assessed from the might be indicated by calc-alkaline metagranitoids of late Central European segment (Rhenish Massif and N parts of Devonian age at the SE margin of the Tepla-Barrandian the Bohemian Massif). The S flank of the belt is best pre- (KOSLER et al., 1993). served in France and N Iberia.

9 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at

Text-Fig. 1. Geological map of the central European Variscides.

Text-Fig. 2. Plate kinematic cartoon for the central European Variscides, from late Silurian to late Devonian times.

References

Unless cited otherwise, the results summarized above are available in DALLMEYER et al. (eds., 1995).

BELKA, Z., AHRENDT, H., FRANKE, W. & WEMMER, K. (1997): Proven- FRANKE, W. & ONCKEN, O. (1995): Zur prädevonischen Geschichte ance of clastic material in the Cambrian and Devonian rocks of des Rhenohercynischen Beckens. – Nova Acta Leopoldina, NF Palaeozoic terranes of southern Poland: evidence from K/Ar 71, Nr. 291, 53–72. ages of detrital muscovites. – Terra Nostra, 97/5 (in press). KEMNITZ, G., DÖRR, W., FIALA, J., VEJNAR, Z. (in press): Gondwana breakup and the northern margin of the Saxothuringian belt. – DALLMEYER, R.D., FRANKE, W. & WEBER, K., eds. (1995): Pre-Perm- Geol. Rundsch. ian Geology of Central and Eastern Europe, 604 p., Springer KOSLER, M., AFTALION, M. and BOWES, D.R. (1993): Mid–late Devo- (Berlin – Heidelberg – New York). nian plutonic activity in the Bohemian Massif: U-Pb zircon iso- DEKORP & OROGENIC PROCESSES RESEARCH GROUPS: The structure topic evidence from the Stare Sedlo and Mirotice gneiss of the Saxonian Granulites – geological and geophysical con- complexes, Czech Republic. – N. Jb. Miner. Mh., 1993/9, straints on the exhumation history of HP/HT rocks. – Tectonics 417–431. (in press). OLIVER, G.J.H., CORFU, F. & KROGH, T.E. (1993): U-Pb ages from DÖRR, W., FIALA, J., VEJNAR, Z. & ZULAUF, G. (1995): U-Pb ages southern Poland: evidence for a Caledonian suture zone be- and structural development of metagranitoids of the Teplá cry- tween Baltica and Gondwana. – Journal of the Geological So- stalline complex: evidence for pervasive Cambrian plutonism ciety, London, 150, 355–369. within the Bohemian massif (Czech Republic). – Geol. ONCKEN, O. (1997): Transformation of a magmatic arc and an oro- Rundsch., 87, 135–149. genic root during oblique collision and its consequences for DÖRR, W., FLOYD, P.A. & LEVERIDGE, B.E. (1999): U-Pb ages and the evolution of the European Variscides (Mid German Crystal- geochemistry of granite pebbles from the Devonian Menaver line Rise). – Geol. Rundschau, 86/1, 2–20. Conglomerate, Lizard peninsula: provenance of Rhenohercy- RAUMER, J.F. VON & NEUBAUER, F., eds. (1993): Pre-Mesozoic nian flysch of SW England. – Sed. Geol., 124, 131–147. Geology in the Alps. – Pre-Permian Geology of Central and Ea- stern Europe, 677 p., Springer (Berlin – Heidelberg – New FRANKE, W., ZELAZNIEWICZ, A., POREBSKI, S.J. & WAJSPRYCH, B. York). (1993): Saxothuringian zone in Germany and in Poland: diffe- ROBARDET, M. & GUTIERREZ-MARCO, J.C. (1990): Sedimentary and rences and common features. – Geol. Rundsch. (1993) 82, faunal domains in the Iberian Peninsula during lower Paleozoic 583–599. times. – In: DALLMEYER, R.D. & MARTINEZ-GARCIA, E. (eds.): FRANKE, W. (1993): The Saxonian Granulites: a metamorphic core Pre-Mesozoic Geology of Iberia, 383–395, Springer (Berlin – complex? – Geol. Rundsch., (1993) 82, 505–515. Heidelberg – New York).

SCHÄFER, J., NEUROTH, H., AHRENDT, H., DÖRR, W. & FRANKE, W. TAIT, J.A., BACHTADSE, V., FRANKE, W. & SOFFEL, H.C. (1997): Geo- (1997): Accretion and exhumation at a Variscan active margin, dynamic evolution of the European Variscan Foldbelt: palaeo- recorded in the Saxothuringian flysch. – Geol. Rundsch., 86/3, magnetic and geological constraints. – Geol. Rundsch., 86/3, 599–611. 585–598. SCHÄTZ, M., BACHTADSE, V., TAIT, J., SOFFEL, H.C. & HEINISCH, H. (1996): New palaeomagnetic results from the southern flank of ZELAZNIEWICZ, A. & FRANKE, W. (1994): Discussion on U-Pb ages the European Variscides from the Northern Greywacke Zone, from SW Poland: evidence for a Caledonian suture zone be- E-Alps. – Terr Nostra, 96/2, 165–168. tween Baltica and Gondwana. – J. Geol. Soc. London, 151, SCHÄTZ, M., BACHTADSE, V., TAIT, J. & SOFFEL, H.C. (1997): Palaeo- 1050–1052. magnetic results from lower Devonian sediments of the south- ZULAUF, G., DÖRR, W., FIALA, J., VEJNAR, Z. (1997): Late Cadomian ern Alps. – Terra Nostra, 97/5 (in press). crustal tilting and Cambrian transtension in the Teplá-Barran- dian unit (Bohemian massif, Central European Variscides), – TAIT, J.A., BACHTADSE, V. & SOFFEL, H.C. (1996): Eastern Variscan Geol. Rundsch., 86, 571–584. fold belt: Paleomagnetic evidence for oroclinal bending. – Geology, 24/10, 871–874.

Manuskript bei der Schriftleitung eingelangt am 2. Oktober 1998 ■